The initial concept of a piezoelectric transformer (PT) was proposed by C. A. Rosen, K. Fish, and H. C. Rothenberg and is described in the U.S. Pat. No. 2,830,274, applied for in 1954. Most of the focus since that time has been on work on high voltage applications such as described in U.S. Pat. No. 2,969,512 with variations of the basic Rosen design. Several different piezoelectric transformer designs have appeared over the years to address power limitations and step-down limitations including thickness mode piezoelectric transformers described in U.S. Pat. No. 5,118,982 U.S. and U.S. Pat. No. 5,241,236.
When working with resonant devices, including piezoelectric transformers, it is often desirable to incorporate resonant frequency self-adjustment capability. It is well known that piezoelectric transformers devices are most efficient in power transference into a given load between their resonance and anti-resonance frequencies closely associated with a selected mechanical vibration mode. There are several important reasons why a piezoelectric transformer device might benefit from the ability to self-adjust its operating drive frequency of mechanical excitation of the primary side. These may include variations in load, operating environment conditions, maximizing gain, voltage gain adjustment, and manufacture tolerances. Often associated with such a loss of efficiency is increase in the circulating current as the selected drive frequency and mechanical resonance drift apart as to cause the drive waveform to have a frequency lower than that of the mechanical resonance of the piezoelectric transformers device. The correct design of such primary side control circuitry as to enable self-adjustment of its operating drive frequency is often the main issue in development and realization of any particular switch-controlled piezoelectric or electromagnetic transformer.
It is well known in the literature that, due to their high Qm characteristics, while having very high efficiency, piezoelectric transformers devices exhibit a narrow band that require operation close to a resonance frequency. Best performance is achieved with the frequency of operation in the inductive region slightly above resonance between the resonance frequency and the anti-resonance frequency because the Qm value of piezoelectric transformers devices is very high, but resonant region is very narrow; therefore can be viewed as a voltage controlled current source, the transformer ratio and efficiency are strongly dependent on frequency control, load fluctuation and input voltage variations. Thus the target operating frequency for maximum efficiency or maximum power delivery has a strong dependence on load temperature, ceramic temperature, and supply characteristics. Inability to adjust the operating frequency in response to changes in any of these parameters will reduces the input-output power transfer efficiency of the PT, in some cases dramatically.
The conventional approach to piezoelectric transformer design has been to develop an optimal linear or switching drive circuit that develops an alternating waveform at a frequency intended to maximize a metric such as maximum throughput power or input to output efficiency.
A number of control augmentations have been designed to enable self-adjustment of the operating drive waveform frequency. Prior art imparts four sequentially connected elements to enable self-adjustment of the operating drive waveform frequency: some form of measurement to generate feedback signals, some form of feedback isolation, some form of feedback control and some form of ac generating drive circuit (modulation). These four sequential elements are so designed and configured as to cause a fifth sequential element consisting of a set of power semiconductors or linear amplifier to generate the alternating drive signal to mechanically excite the primary (driven) side of a PT.
Early piezoelectric transformer devices used a combination of a Voltage Controlled Oscillator and analog circuitry. With the advent of silicon processor devices, modern piezoelectric transformer devices in employment now incorporate one or more digital processors to implement the measurement processing, the control strategy and the switching drive control signal generation.
Later, the prior art began employing voltage measurements taken at the secondary side of a piezoelectric transformer as to adjust the resonant drive frequency downward as a function of increasing load as disclosed in U.S. Pat. No. 5,768,111 (the '111 Patent). A voltage measurement system was introduced at the output secondary side of the piezoelectric transformer that provides a signal into an intermediate feedback control circuit. The feedback control circuit incorporates a voltage-controlled oscillator as to generate a control signal as an output that acts as input into a waveform generator that incorporates an adaptive pulse sequence generator. The feedback control circuit provides a commanded variation of the waveform generator as to cause it, in turn, to adjust the gate drive signal at a switchmode block that incorporates a pair of MOSFET switches. The modified output of the switchmode block provides a sinusoidal drive signal that connects to the primary side of the piezoelectric transformer that now better matches the change in mechanical resonance of the piezoelectric transformer due to variation of the output side electrical load. This construction is sometimes referred to as a self-adjusting circuit.
Subsequent approaches to piezoelectric transformer design have sought improvements in the self-adjusting circuit. Modifications of self-adjusting circuits have included replacing the separate oscillator within the feedback control circuit with an internal (self-mechanical) oscillator; replacing the separate oscillator with control modulation schemes such as PWM and PWM/PFM as to more broadly track variations in load or temperature; deriving the signal into the measurement component of the circuit of as output electrical measurements from the primary, deriving the signal into the measurement component of the circuit as output electrical signal measured at the secondary (driven) side of the piezoelectric transformer device or some combination of the electrical load (see FIG. 2); deriving the signal into the measurement component of the self-adjusting circuit as output electrical measurements directly from the secondary (output) side of the piezoelectric transformer and the electrical load (FIG. 3); deriving the signal into the measurement component of the self-adjusting circuit as the as output electrical measurements taken directly from the primary (driven) side of the piezoelectric transformer and the electrical load (see FIG. 4); included replacing the separate oscillator within the feedback control circuit with an internal (self-mechanical) oscillator with feedback controlled oscillator network incorporating a phase differential subcircuit.
Each of the approaches illustrated in FIGS. 1-4 are more specifically described herein. Referring to FIG. 1, a set of electrodes 11 is disposed as part of the secondary output (driven) component of the piezoelectric transformer device 100 which convey electrical signal information to the measurement and processing block 13 via an isolation block 12. The isolation block 12 maintains galvanic isolation of the measurement block 13, feedback control/filter block 14, and waveform generator block 15 from the output. Without this isolation block 12 the circuit implementation will nullify isolated operation of the transformer. The measurement block 13 processes the electrical signals from electrodes 11 and provides them as an input into a feedback control/filter block 14 at the same waveform frequency as that which appears on secondary component 2. The feedback control/filter block 14 subsequently performs calculus on these signals as to provide control inputs into a waveform generation block 15 as to adjust or modify the waveform frequency observed at tap 11. This often consists of the feedback circuit 14 performing calculus on a set of phase differential measurements. The waveform generator block 15 acts as to develop a command signal into the power circuit 16 as to generate an output from power circuit 16 that is a repetitive waveform of the adjusted frequency. This waveform is commonly a pulse train at the new adjusted frequency. The dc voltage level Vcc 21, of fixed amplitude, acts as an input into power circuit 16. Power circuit 16 consists of some components, usually power semiconductor switches or a linear amplifier as to act upon this dc voltage input as to produce an output at 17 that is a repetitive ac waveform of the adjusted waveform frequency commanded by waveform generator 15. The output 17 acts as an electrical input to a passive circuit 18 disposed between the power circuit 16 and the primary (input) side 1 of the piezoelectric transformer device 100. Commonly this passive circuit provides a conversion from pulse generation to sinusoid of the same frequency and amplitude. Such passive circuit block has also commonly been used to enable a desirable circuit topology such as Zero Voltage Switching (ZVS) operation. The outputs of the passive circuit 18 provide a voltage waveform at the adjusted frequency. The voltage waveform is electrically connected to sets of electrodes 7a and 7b disposed integral to the primary (input) side 1 of the piezoelectric transformer device 100 as to induce fluctuations in electrical field at the same adjusted waveform frequency as commanded by the waveform generator 15. The electrodes are so configured as to utilize the converse effect of piezoelectric material. The converse effect causes an internal stress cycle at the adjusted waveform frequency as commanded by the waveform generator 15. The stress cycle mechanically couples with a secondary (load output) component 2 of device 100 via an electrical isolation region 6. Region 2 incorporates sets of electrodes that use the converse effect of the induced stress cycle as to generate a sinusoidal voltage at locations 11. The potential difference between the locations 11 and ground represent a voltage that is a very small phase shift of the operating mechanical frequency of the device 100. Location 11 circuit elements may be an independent and galvanically isolated from load output 20 connections 9 and 10 incorporated in device 100, or these may be directly connected to one or more of the load output connections.
Referring to FIG. 2, the operation is very similar to that described for FIG. 1 with the primary distinction being that measurement block 12 is the additional recipient of a set of parametric electrical measurements, such as load, current, phase 19b taken at some circuit locations 19c in the load circuit. Parts of measurement block, control block and drive block are commonly subsumed by a custom controller microchip 101.
FIG. 3 illustrates a variation of the extrinsic circuit strategy of the prior art to enable self-adjusting frequency waveform drive signal on the primary (input) side of the piezoelectric transformer that uses electrical parameter measurements taken at both primary and secondary sides of the piezoelectric transformer device. The drive side 1 now consists of two separate components 1a and 1b that may, or may not, be electrically isolated. Due to the direct piezoelectric effect, component 1a generates a charge waveform of the same frequency as that of waveform generator 15. Locations 19a and 19b are in this case recipients of a set of electrical signals from the second component of the primary 1a and the secondary (output) component 2. Parts of measurement block, control block and drive block are commonly subsumed by a custom controller microchip 101.
The prior art has exemplified circuits that are a combination of measured parametric outputs of FIG. 2 and FIG. 3 along with introducing additional subcircuits such as phase differential control subcircuitry or secondary voltage regulation switch subcircuits as to enhance some intended performance metric such as throughput power, output regulation or maximum efficiency. All such prior art builds upon, and subsumes, the sequential circuit layout of FIG. 1.
Referring to FIG. 4, the circuit introduces an electrically isolated tertiary component 3 of piezoelectric transformer device 100 that is isolated both from the primary 1 and secondary (output) 2. A set of electrodes are disposed as integral to the tertiary 3a as to generate a charge when mechanically excited. This charge generation by the electrodes provides an output set of electrical parameters that act as input signals to a feedback circuit 14. The common frequencies of the output electrical parameters of these signals at 5a are sinusoidal and approximate to the value of the operational mechanical mode of the piezoelectric transformer. The feedback circuit 14 directly performs calculus on these incoming signals and provides resultant (analog or digital) output signal transferred as input at the waveform generator 15 as to appropriately adjust the drive frequency at the set of electrodes inclusive in component 1. The voltage waveform is electrically connected to sets of electrodes 7a and 7b disposed integral to the primary (input) side 1 of the piezoelectric transformer device 100 as to induce a fluctuations in electrical field at the same adjusted waveform frequency as commanded by the waveform generator 15. The electrodes are so configured as to utilize the converse effect of piezoelectric material. The converse effect causes an internal stress cycle at the adjusted waveform frequency as commanded by the waveform generator 15.
The stress cycle mechanically couples with a tertiary output subcomponent 3 of device piezoelectric transformer 100 via electrical isolation regions 6a and 6b. Regions 6a and 6b are constructed as to cause region 3 to be electrically isolated from the primary (input) component 1 and the secondary (output) component 2 of the piezoelectric transformer 100. Region 3 incorporates sets of electrodes that use the converse effect of the induced stress cycle as to generate voltages at locations 5a and 5b. The potential difference between the 5a and 5b represents a sinusoidal voltage waveform, or sinusoidal current that is not referenced to the ground reference of either the primary component 1 or the secondary component 2. This is advantageous in that this potential is insensitive to variations in the load potential difference between 9 and 10. The sinusoidal voltage represents a small phase shift of the actual operating mechanical frequency of the device 100. The feedback circuit acts to provide a calculated input signal into a resonant drive circuit comprised of waveform generator 15 and drive circuit 16 consisting of power semiconductor switches. The output of a resonant driver circuit consists of a pulse train at the commanded frequency as developed by the Feedback feedback circuit. piezoelectric transformer devices do not withstand hard switching drive waveforms, thus, in order to protect from damage to the piezoelectric transformer device it will be necessary to adjust the hard switched pulse train output of the driver circuit by disposing a passive circuit 18 between the driver circuit and the input voltage waveforms at the sets of electrodes 7a and 7b. 
All such modifications as depicted in FIGS. 1-4, along with the designs incorporated into all their references, that seek to improve the self-adjusting drive waveform piezoelectric transformer devices of the '111 Patent provide for the same sequential circuit topology connectivity as the '111 Patent. That being the sequential or series connection of a feedback measurement block, isolation block, feedback control block and waveform generator (modulation) block as to drive a set of power semiconductors or linear amplifier. Such ‘extrinsic’ piezoelectric transformer circuits provide for a self-adjusting voltage waveform excitation of the primary side of a PT device, as shown in FIG. 1, that adjusts as to reflect the updated value of the mechanical excitation design point of interest. Because of the employment of such multiple function blocks within the piezoelectric transformer circuit needed to achieve correct adjustment of the resonant frequency drive signal, devices of prior art entail difficulties with cost, complexity, part count, and digital component usage
Some prior art approaches have sought to employ a replacement configuration for the standard controlled power switch sinusoidal voltage waveform primary side excitation. This approach instead seeks to use a feedback-controlled oscillator network to generate a self-adjustable frequency variable drive. This approach nevertheless still incorporates all four elements of measurement block, measurement isolation block, measurement control block and drive circuit block. Thus, in such an approach, the measurement, measurement control, and drive circuit block must also maintain this galvanic isolation from the output or the feedback implementation will nullify isolated operation of the transformer.
There have been efforts to improve the basic circuit design of the '111 Patent and its modifications by introducing (galvanic) isolated tertiary components into the piezoelectric transformer construction as to provide the direct signal generation that does not emanate from the primary (drive) side of the piezoelectric transformer, secondary (output) side of the piezoelectric transformer or the electrical load. The advantage of the design is it bypasses any need for a measurement block and, in some cases, the need for an isolation block. The introduction of an isolated measurement enables improvements in the overall performance and efficiency. This prior art a controlling drive circuit to adjust frequency in response to variations in load or temperature takes as input the outputs of one or more isolated tertiary components of the piezoelectric transformer and then uses these as direct signal inputs into the feedback control circuit of the extrinsic circuit. The direct signal input extrinsic circuit modification is shown in FIG. 4.
Complex circuitry is required in order to enable tracking and adjustment of the resonant frequency, or equivalent metric. The known extrinsic circuits require a measurement block, isolation block, feedback control block and waveform generation (PWM/PFM modulator or oscillation controller), or some combination thereof, to take an output signal and drive a power switch or linear amplifier block as to enable self-adjusting waveform excitation at the primary side of a piezoelectric transformer device. This type of extrinsic circuit solution to the design of variable frequency drive piezoelectric transformers can be sensitive, complicated and costly. This leads to simpler designs that only operate at fixed frequency with the goal to modify the drive waveform generator circuit as to maximize the operational region over which it is effective. However, fixed frequency drive PTs now also require a secondary reference resonance source on the primary side of the transformer. A significant dilemma is that fixed frequency drive piezoelectric transformer devices of prior art cannot tolerate significant changes in load, thermal or supply conditions whereas variable frequency drive introduce complexity, cost and failure modes.